A general purpose valve may work for what you are trying to model. This allows you to include a headloss curve where you can enter a table of flow and headloss, which allows you to control the headloss at the valve.

For information on how valves operate in HAMMER, please see this link: Model Reference - Valves. You can include an operating rule to change how the valve operates, but the valve headloss is assumed to be held at the value calculated during the initial conditions calculation.

In WaterGEMS (or with a steady state or EPS in HAMMER initial conditions), you can model a specific rating table of flow vs. headloss with the GPV element, but if you want a constant headloss for all flows, use a PBV (pressure breaker valve).

Note though that these will not work the same during a transient simulation in HAMMER. HAMMER always uses the pair of initial flow and initial headloss through each valve element, to calculate a discharge coefficient which it uses during the transient simulation. This means that even if you use a PBV for the initial conditions, the headloss will vary during the transient simulation.

I understand that the GPV can represent alinear headloss (Darcy-like) in Watergems, but that Hammer will use a quadratic headloss calibrated on the initial configuration. The difference will be around 1 bar in my system during the shutdown transient (at half discharge, the headloss in the real system will be 1/2 of the initial headloss; 1/4 following the quadratic assumption used by Hammer). If not any other idea, I will have to make some sensitivity analysis to understand the incidence of this assumption on the design of the surge protection.

A sensitivity analysis would allow you to see how much of an impact the valve settings will have. Reviewing the Modeling Reference related to valves is also recommended. In HAMMER, the initial conditions calculation will calculated a Darcy-Weisbach friction factor. This value is used throughout the transient run.

If the valve is initially partially closed, you can also review this link, which has some details how to model this in HAMMER.